Ion-conducting inorganic oxide ceramic materials of certain compositions transport or permeate ions at high temperatures, and this phenomenon is the basis for practical applications in fuel cells, gas analysis and monitoring, and the separation of gas mixtures. In a number of such practical applications, oxygen ions migrate as current under an imposed potential gradient through an oxygen ion-conducting electrolyte from the cathode side, where oxygen ions are generated by reduction of oxygen or other gases, to the anode or oxygen side, where the oxygen ions are consumed to form oxygen or other gases.
Oxygen ion-conducting solid electrolytes can be constructed in tubular, flat plate, and honeycomb or monolith multicell configurations. The flat plate configuration, in which a plurality of planar electrolytic cells are stacked to operate in electrical series, is favored in many applications for ease of assembly and compact dimensions. The practical application of ion conductor systems for gas separation, regardless of design configuration, requires that the cells operate under a differential pressure and/or differential gas composition between the feed side (cathode) and the permeate side (anode). In the separation of oxygen from an oxygen-containing gas, for example, the pressure and/or gas composition of the oxygen-containing feed gas and the oxygen-depleted discharge gas (also defined as nonpermeate gas) can differ from the pressure and/or gas composition of the oxygen produced at the anode (also defined as permeate gas), depending on the stack design and product requirements. Gas-tight seals between selected structural components of the system therefore are required to maintain product gas purity, whether the product is oxygen-depleted discharge gas or high purity oxygen produced at the anode.
An oxygen ion-conducting system having a disk or planar stack configuration is described in U.S. Pat. No. 4,885,142 in which an oxygen-containing gas is introduced through axial feed ports, flows radially across stacked electrolyte disks, and is discharged through an axial discharge port preferably centrally located. Oxygen product is withdrawn through a separate series of axial discharge ports. The feed and product gases are segregated by interfitting parts of the disk assembly, which is described to form a substantially sealing relationship. The stack operates with no differential pressure across the cells and the use of sealants is not disclosed. A similar system is disclosed in papers by J. W. Suitor et al entitled "Oxygen Separation From Air Using Zirconia Solid Electrolyte Membranes" in Proceedings of the 23rd Intersociety Energy Conversion Conference, Vol. 2, ASME, New York, 1988, pp. 273-277, and by D. J. Clark et al entitled "Separation of Oxygen By Using Zirconia Solid Electrolyte Membranes" in Gas Separation and Purification, 1992, Vol. 6, No. 4, p. 20-205.
U.S. Pat. No. 5,186,806 discloses a planar solid electrolyte cell configuration in which alternating plates and gas distribution support members are stacked in series. In one configuration the plates are made of ion-conducting non-porous material and the support members are made of non-porous electrically-conducting material. A series of ports and bosses in the support members coincide with ports in the electrolyte plates to yield a flow configuration in which feed air flows radially across the cathode sides of the electrolyte plates in an inward direction, and the oxygen-depleted air is withdrawn axially through a centrally-located conduit formed by congruent ports in the electrolyte plates and support members. Oxygen formed on the anode sides of the electrolyte plates flows radially outward and is withdrawn through a plurality of axial conduits formed by separate congruent ports in the electrolyte plates and distribution members. Sealing between the oxygen side and feed gas sides of the stack components according to the disclosure is accomplished by direct contact between electrolyte plates and flat bosses on the support members, and also at the stack periphery by contact between continuous flat raised rings on the support members and the flat electrolyte plates. No sealant is described in the seal regions formed by direct contact between regions of the support members and the electrolyte plates. The seal regions formed by the bosses in contact with the anode side and the cathode side of each electrolyte plate are radially and circumferentially offset, but the corresponding peripheral seals are congruent or directly opposed.
An ion conducting device having a plurality of electrolyte plates in a stacked configuration is disclosed in U.S. Pat. No. 5,298,138 in which supporting electrically-conducting interconnects are not used. The electrolyte plates are separated by alternating spacers made of electrolyte material which is attached near the edges of the plates by glass sealant to allow crossflow feed. While this stack design is simplified by eliminating interconnects, the electrolyte plates are not supported in the central region, which allows operation only at very small pressure differentials between the anode and cathode sides of the cells.
European Patent Application Publication No. 0 682 379 A1 discloses a series planar electrochemical device for gas separation in which alternating electrolyte plates and electrically-conducting interconnects are assembled in a stack configuration. The anode and cathode in electrical contact with opposite sides of each electrolyte plate are radially coextensive, i.e. congruent. The interconnects contain channels designed such that feed gas flows through the cathode side in crossflow mode and oxygen formed on the anode side is withdrawn in a crossflow mode in a flow direction perpendicular to the feed gas flow. The interconnects and electrolyte plates are connected by glass sealing areas parallel to the channels in the interconnects. Portions of the anode and cathode seals are directly opposed on each electrolyte plate.
A technical report entitled "Stacking Oxygen Separation Cells" by C. J. Morrissey in NASA Tech Brief, Vol. 15, No. 6, Item #25, June 1991 describes planar stacked electrolyte cells comprising alternating electrolyte plates and gas distribution interconnects. The anode and cathode on each electrolyte plate are directly opposed across the electrolyte plate, i.e., they are congruent. Glass seals are used between each electrolyte plate and the adjacent interconnects, and the seals are directly opposed across the electrolyte plate, i.e., they are congruent. This design includes a nonporous electrically insulating layer located at the edge of the stack between the interconnects.
Planar stacked electrolyte cells comprising alternating electrolyte plates and interconnects having embossed gas passageways are disclosed in a technical report entitled "Thinner, More Efficient Oxygen Separation Cells" by C. J. Morrissey in NASA Tech Brief, Vol. 17, No. 4, Item #100, April 1993. Air is introduced into the cells through axial manifolds passing through the stack which provide feed in radial flow across the cathode sides of the cells. Oxygen-depleted air is withdrawn through a centrally-located axial manifold. Oxygen product from the anode sides of the cells is withdrawn through additional axial manifolds passing through the stack at locations circumferentially disposed between the air feed manifolds. The anode and cathode on each electrolyte plate appear to be directly opposed across the electrolyte plate, i.e., they are congruent. While seals are not specifically discussed in the text, it appears from the drawings that seals between each electrolyte plate and the adjacent interconnects would be directly opposed across the electrolyte plate, i.e., they are congruent.
Thus the state of the art in the design of stacked ion-conducting electrolytic cells teaches methods for sealing the anode and cathode sides of the cells to prevent cross-contamination of feed and product gases. Sealing has proved difficult, however, at the high temperatures and electrochemically active conditions encountered in these systems. The practical application of ion conductor systems for gas separation, regardless of design configuration, requires that the cells operate under differential pressures and/or differential gas compositions between the feed and product sides of the cells, and this in turn requires robust gas-tight seals between the stack components. This need is addressed in the invention which is described in the specification below and defined by the claims which follow.